Microreactor-type chemical processing units have been proposed where fluids (liquids or gases) are guided in etched, molded, drilled or otherwise formed fluid channels in or on planar substrates. Fluid channels are patterned with elementary fluidic structures (e.g., mixers and residence time segments) to form circuits that provide more complex chemical processing functions. Planar substrates can be stacked to extend functionality in a single reaction unit, providing a modular chemical processing system that can target multiple applications.